The present application is based on Japanese Patent Application No. 2000-337729, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical element for use in forming a liquid-crystal display device in which hue is changeless in spite of an azimuth angle changed at oblique viewing, and an illuminator using the optical element.
2. Description of the Related Art
A group to which the present inventors belong has already proposed an optical element having a function of separating incident light into polarized light components through reflection and transmission, wherein variation in transmittance and hue from in-plane averages ranges from 0.5 to 5.0 in terms of xcex94E given by the Hunter""s color difference formula: xcex94E={(xcex94L2+xcex94a2+xcex94b2) (in which L is a lightness index, and a and b are chromaticness indices) and wherein the optical element may be constituted by a laminate of a reflective type polarizer provided with a quarter-wave plate and a dichromatic polarizer as occasion demands (Unexamined Japanese Patent Publication No. Hei. 11-311710). This proposal was aimed at applying an optical element satisfying xcex94E to a liquid-crystal display device to attain improvement of luminance and prevention of display unevenness. There was, however, a disadvantage in that hue varied largely in accordance with the azimuth angle changed at oblique viewing though good display could be obtained at frontal viewing.
There has been another proposal for a liquid-crystal display device in which variation of chromaticity between hue at frontal viewing and hue at oblique viewing is suppressed (Unexamined Japanese Patent Publication No. Hei. 10-319235). This proposal cannot cope with variation of hue in accordance with the azimuth angle changed at oblique viewing. Even if chromaticity at an oblique viewing point having an azimuth angle of 0 or 45 degrees and an elevation angle of 70 degrees could be suppressed to 0.04 which is a value equal to that at a frontal viewing point, the chromaticity changes by 0.08 and the hue varies largely in accordance with the azimuth angle changed from 0 to 45 degrees when both the chromaticity coordinates x, y change in a plus direction in the case of an azimuth angle of 0 but change in a minus direction in the case of an azimuth angle of 45 degrees. Accordingly, this measure cannot sufficiently cope with variation of hue in accordance with the azimuth angle changed at oblique viewing when the direction of coloring turns over between plus and minus directions in accordance with the azimuth angle at oblique viewing.
An object of the invention is to develop an optical element for use in forming a liquid-crystal display device in which hue is changeless in spite of the azimuth angle changed at oblique viewing, and an illuminator using the optical element.
According to the invention, there is provided an optical element having: a laminate including a reflective type polarizer for separating incident light into polarized light components through reflection and transmission, and a dichromatic polarizer, wherein a variation of hue between two points calculated by the expression: {(x2xe2x88x92x1)2+(y2xe2x88x92y1)2} is not larger than 0.06 when x1 and y1 express a hue at a point with a predetermined elevation angle and any azimuth angle, and x2 and y2 express a hue at another point with the same elevation angle and a changed azimuth angle in the case where transmitted light is obliquely viewed in the condition that the laminate is disposed on a planar light source so that the reflective type polarizer is located in an inner side. There is further provided an illuminator having a planar light source including a reflection layer on its rear surface side, and an optical element defined above and disposed on the front surface side of the planar light source so that the reflective type polarizer of the optical element is located in an inner side. In addition, there is provided a liquid-crystal display device having an illuminator defined above, and a liquid-crystal cell disposed on a light exit side of the illuminator.
The optical element or illuminator according to the invention can be used for forming a liquid-crystal display device in which hue is changeless in spite of the azimuth angle changed at oblique viewing. Hence, a liquid-crystal display device good in display quality and having a large-scale screen can be formed advantageously.